1. Field of the Invention
The present invention relates to digital-to-analog converters, and more particularly to a digital-to-analog converter of a current segmentation type.
2. Description of the Prior Art
A general current output digital-to-analog (D/A) converter obtains a desired analog current by the combination of a plurality of current sources. FIG. 1 illustrates such a current output D/A converter. As shown, an analog output current I.sub.out is determined by the combination of current sources I.sub.1, I.sub.2, . . . I.sub.N. An output voltage V.sub.out across both terminals of an output resistance R.sub.out is equal to a multiplication of the output resistance and output current as represented by the following: EQU V.sub.out =I.sub.out .multidot.R.sub.out ( 1)
Actually, each current source has a finite output resistance as shown in FIG. 2. The output resistance varies with the state of a current switch. If the current source is connected to an output terminal by the switch, i.e. during the ON state, a resistance R.sub.I of the current source appears as an equivalent resistance at the output terminal. If the current source is separated from the output terminal, i.e. during the OFF state, the equivalent resistance has an infinite value. Accordingly, it will be readily appreciated that the equivalent resistance at the output terminal varies with the state of the current switch. When k switches are in the ON state, the equivalent resistance R.sub.OUT.sup.TOTAL (K) seen from the output terminal is given by: ##EQU1## where R.sub.out.sup.i is the output impedance of the i-th current source.
The output current I.sub.out is determined by: ##EQU2## where I.sub.i is the output current of the i-th current source.
If R.sub.OUT.sup.TOTAL (K)R.sub.L, the equation (3) can be represented by the following equation (4) to maintain the linear relationship between the output current and an input code, and otherwise linearity deviates from an ideal value. ##EQU3##
However, a conventional D/A converter uses a cascode arrangement, or an additional transistor with a long length to achieve high impedance for satisfying the equation (4). The former has a disadvantage in that a voltage operation of an additional transistor is difficult. The latter increases the size of the transistor to unnecessarily consume power and lower the speed of the device, due to an increased parasitic capacitance. A variation in the equivalent resistance results in a nonlinearity between an input digital code and analog output current. Consequently, the integral nonlinearity (INL) of the D/A converter is increased. This disadvantage causes a serious problem with an increase in the number of bits of the D/A converter.
A general process for overcoming this problem is to reduce an effect upon the output current by considerably increasing the output resistance during the ON state of each current source. In order to increase the output resistance of each current source, the size of the device is increased, or a circuit for increasing the output resistance is used. The former increases the total size of the devices and lowers the speed of the device due to the parasitic capacitance. The latter is difficult to use with a low supply voltage.